The present invention is directed to electric and hybrid-electric vehicles, which rely on electric motors and drives for propulsion and braking assistance. The vast majority of these vehicles rely on battery packs with optimized energy density as the primary energy storage system. As battery driven systems become more prevalent, engineers are beginning to discover limitations with a battery only system. One major limitation is that the longevity of the battery is drastically affected by transient loads that are applied during acceleration and regeneration. Another limitation is that batteries do not efficiently recapture energy during regeneration. The second limitation reduces the usable range of an electric vehicle and greatly reduces the effectiveness of a hybrid vehicle. Both of these limitations can potentially be addressed through the use of a secondary source of energy that can more effectively provide short bursts of power for acceleration and quickly recapture energy during regenerative braking. An example of such a secondary energy storage system is an ultra-capacitor. Ultra-capacitors are a new type of capacitor, which has significantly greater energy storage capability over a traditional capacitor. Pairing an ultra-capacitor with a battery is not the only possible embodiment of the invention. Other energy storage systems can be paired where one has most of the energy capacity and the other has most of the power capacity. In addition, energy storage systems could be paired for purposes other than splitting between high energy and high power. For example, if a vehicle has two removable battery packs, it might be advantageous to deplete one battery pack before the other so the empty pack could be replaced mid-trip. For simplicity's sake, this document uses a battery as the primary storage and an ultra-capacitor as the secondary storage device used for power.
As designers begin to investigate the use of ultra-capacitors in conjunction with batteries, they quickly discover it is a challenging task. Because batteries operate over a very narrow voltage range, while capacitors must operate across a broad voltage range, it is not practical to simply connect the capacitors and batteries in parallel. What is required is some power electronic device, which allows the motor drive to direct power to either the batteries or the capacitors at will, while allowing the batteries and capacitors to work at different voltages. The conventional solution to this problem is to use a DC to DC converter to adapt the voltage range of the capacitors to that of the batteries, while causing power to flow in either direction under some form of control. FIG. 1 shows the major components of a typical prior art system in which a DC to DC converter is used to couple an ultra-capacitor to a battery bank and motor drive.
There are two major problems with a DC to DC converter based power sharing scheme. The first problem is that the aforementioned type of DC to DC converter does not exist in the market today. The industry has not matured yet. As a result, converters are prohibitively expensive and require a substantial up-front investment of non-recurring engineering costs. Contrast this with the motor drive market, which is quite mature. There exist today a large number of off the shelf, cost optimized motor drives from which a designer may select. The second major problem with a DC to DC converter approach is that efficiency is lost in the power conversion process. In any motor drive system, some amount of efficiency is lost as power passes through the motor drive in order to power the motor. With a DC to DC converter, a second power conversion process takes place for one of the two power sources. This power conversion causes an additional loss of efficiency for the associated power source.